Research communities and microelectronics industries have known about micro-fabricated charged particle emission devices for some time. While there are many types of charged particle emission devices, each type has at least one emitter from which the charged particle emission device can emit electrons or ions, depending upon its type. Some charged particle emission devices use liquid metal as a charged particle source. The liquid metal flows through a channel in the emitter to the emission site, such as the open end of a capillary or an exposed needle-like projection. An applied voltage in excess of a threshold relative to the emission site induces liquid metal at the site to ionize and the emitter to emit charged particles. A further increase in voltage induces a corresponding increase in the emitted charged particles, whereas when the voltage falls below a corresponding threshold, the emitter ceases to emit charged particles.
Because of their microscopic scale of geometries, micro-fabricated charged particle emission devices require relatively low power to emit charged particles efficiently. For instance, the operating voltage for inducing charged particle emission from an emitter tip of a gated charged particle emission device can range between 50 and 100 volts for an electron source and between 500 and 1000 volts for a liquid metal ion source. Consequently, micro-fabricated charged particle emission devices have found use in a variety of applications, such as ion thrusters, micro-fluidic dispensers, and satellite charge controllers.
The microscopic scale of geometries, however, also poses a problem for those charged particle emission devices using liquid metal as a charged particle source: if the liquid metal does not wet the surfaces of the emitter channels properly, it cannot overcome the forces that resist its flow into and through such channels or surfaces. As a result, the liquid metal is unable to readily and reliably flow sufficiently near the emission site in order to be ionized, and the emitter cannot then operate effectively as a charged particle emitter.
Some techniques attempt to improve wetting and the flow of the liquid metal by heating the charged particle emission device. Such heating, however, can cause material to evaporate and coat the emitting structures. For arrays of densely packed emitters and their microscopic geometries, the unwanted coating can have deleterious consequences, such as electrical leakage and shorting. Moreover, some materials used to construct the arrays are unable to withstand the high temperatures sometimes used to improve wetting. There is, therefore, a need for a method and materials to assist in wetting the microscopic channels or along the surfaces of the emitters without the aforementioned disadvantages.